1. Field of the Invention
The invention relates to phase lock loops and more particularly to a narrow band phase lock loop that possesses a wide input frequency acquisition range.
2. Background of the Invention
Information is transmitted by radio communication systems within frequency channels (bands) about a carrier frequency that is assigned to each channel. These systems require stable frequency sources for the carrier frequency to achieve channel selectivity and a minimum of interchannel interference. At the lower frequencies, i.e., one MHz, this does not pose a serious problem. At these frequencies, oscillators are readily obtainable with stabilities that maintain a selected carrier frequency within a small percentage thereof thus providing an operating frequency range that permits non-interfering channelized communications. At very high frequencies, VHF, (30-300 MHz), however, these same percentage stabilities may provide carrier frequencies in a proportionately wider frequency band thus adversely affecting carrier selectivity and interchannel interference. Consequently, VHF sources are generally synthesized from a highly stable frequency standard such as a precision crystal oscillator.
Crystal oscillator frequency synthesis may involve one or more precision crystal oscillators and a plurality of mixers, arranged to obtain and combine harmonics of the oscillators to make available a multiplicity of output signals harmonically related to the crystal oscillator sources. An undesirable concomitant of this method is the generation of spurious frequencies in the combining mixers. Selection of the desired frequency combination, while simultaneously rejecting undesired harmonics and spurious frequencies, is a difficult problem which requires highly selective filtering, the problem becoming more difficult as the channel frequency spacing is decreased. A filtering method to reject the undesired frequencies may comprise a phase lock loop wherein a voltage controlled oscillator (VCO) is referenced to a selected synthesized carrier frequency. In these loops, the VCO output frequency is generally fed through a narrow band filter, to reduce noise caused by spurious frequencies, and other sources, then to a phase detector wherein it is compared to the synthesized carrier frequency. Resulting error signals are applied from the phase detector through the lowpass filter to control and correct the frequency of the VCO and thereby provide a substantially spurious free output carrier. The bandwidth required for the elimination of spurious frequencies however, limits the acquisition range of the loop and consequently the synthesized carrier frequencies to which the VCO may be locked. What is desired is a loop that exhibits wideband acquisition and narrow band filter characteristics in-lock.